1. Field of the Invention
The present invention relates, in general, to 4-stage automatic transmissions used for electronically changing the speed of a driven shaft of an automobile between four stages and, more particularly, to a hydraulic control device for such transmissions capable of performing an R-N-D shifting operation without generating any shifting shock, thus immediately and smoothly performing such an R-N-D shifting operation.
2. Description of the Prior Art
As well known to those skilled in the art, an automatic transmission for automobiles is comprised of a torque converter, a change gear mechanism and a hydraulic control circuit. The change gear mechanism is connected to the torque converter, while the hydraulic control circuit selectively actuates the gears of the change gear mechanism in accordance with running conditions of a car.
In the torque converter, an impeller is directly connected to the crank shaft of an engine, while a turbine is directly connected to the driving shaft of the transmission and is placed opposite to the impeller. In the operation of the torque converter, pressurized oil is fed from the impeller to the turbine, thus transmitting kinetic energy to the turbine prior to being returned to the impeller. In order to change the flowing direction of return oil and increase torque of the impeller, the torque converter also includes a stator.
In the operation of the automatic transmission, engine output power is primarily transmitted to the impeller of the torque converter and is secondarily transmitted to the change gear mechanism through the turbine of the converter. The change gear mechanism is an epicyclic gear train, which is comprised of a sun gear, a ring gear and a gear carrier. When the gears of the epicyclic gear train are appropriately controlled, they convert the torque from the turbine into a speed change ratio due to their relative rotating motions prior to transmitting the torque to the driven shaft of the transmission.
In order to control the gears of the change gear mechanism, the transmission also includes a plurality of frictional members, such as a front clutch, a rear clutch, a low-reverse brake, an end clutch and a quick-down brake. The above frictional members are actuated by pressurized oil output from an engine-operated oil pump.
When a plurality of sensors, which are mounted to predetermined parts of a car, apply signals, indicative of running conditions of the parts, to a TCU (Transmission Control Unit), the TCU operates the signals and controls a plurality of solenoid valves. The solenoid valves are mounted to the oil passages extending from the oil pump to the frictional members, thus controlling pressurized oil for the frictional members under the control of the TCU. When the oil pressure in the oil passages is partially changed by the solenoid valves, a plurality of valves of the oil passages are operated to control the oil pressure acting on the frictional members. Therefore, the frictional members control the gears of the change gear mechanism in accordance with the oil pressure acting on the frictional members.
FIG. 6 shows the construction of a typical hydraulic control device for 4-stage automatic transmissions.
As shown in the drawing, the typical hydraulic control device for automatic transmissions includes a manual valve 52, an SCV (Shift Control Valve), a reducing valve 53 and a PCV (Pressure Control Valve). The manual valve 52 is operated in conjunction with a gear shifting lever and receives pressurized oil from both an oil pump 50 and a regulator valve 51. The SCV selectively generates an oil pressure corresponding to one of the 1st to 4th stages in response to the pressure from the manual valve 52, thus setting a desired stage. The reducing valve 53 generates a stable reference, control pressure for controlling the valve spools of the valves, which are controlled by solenoid valves. Meanwhile, the valve spool of the PCV is controlled by the control pressure of the reducing valve 53 so that the PCV generates a control pressure for allowing the frictional members of the change gear mechanism to be smoothly operated during a shifting operation.
The hydraulic control device also includes two SCSVs (Shift Control Solenoid Valves): an SCSV-A and an SCSV-B, and a PCSV (Pressure Control Solenoid Valve). The two SCSVs commonly control the SCV under the control of the TCU, while the PCSV controls the PCV.
An N-D control valve 54 is to prevent any shifting shock during an N-D shifting action of the gear shifting lever, while an N-R control valve 55 is to prevent any shifting shock during an N-R shifting action of the gear shifting lever. The hydraulic control device also has an 1-2 shift valve 56. The valve spool of the 1-2 shift valve 56 is operated by either the 2nd-stage or reverse-stage pressure applied thereto through the 2nd-stage oil passage of the SCV, thus controlling the oil passages for both the SA (Servo Apply) part of the quick-down brake and the low-reverse brake.
The hydraulic control device further includes a 2-3/4-3 shift valve 57, of which the valve spool is connected to the 3rd and 4th-stage oil passages from the SCV at its both ends. In a 3rd-stage, the valve spool of the above valve 57 is operated by the 3rd-stage pressure of the 3rd-stage oil passage, thus allowing the control pressure of the PCV to pass through the 1-2 shift valve 56 prior to controlling both the SR (Servo Release) part of the quick-down brake and the front clutch. Meanwhile, in a 4th-stage, the valve spool of the shift valve 57 is operated by the 4th-stage pressure of the 4th-stage oil passage, thus allowing the control pressure to be released from both the SR part of the quick-down brake and the front clutch through the valve 57.
The rear clutch is controlled by a spring-biased RCEV (Rear Clutch Exhaust valve). In a 1st, 2nd or 3rd-stage with the gear shifting lever being positioned in the D-range, the RCEV is biased by its spring installed at one end thereof, thus applying a drive pressure to the rear clutch. Meanwhile, when a 4th-stage pressure acts on the opposite end of the RCEV, the RCEV stops the pressure for the rear clutch and allows the pressure of the rear clutch to be released. When the 4th-stage pressure is removed from the RCEV, the valve spool of the RCEV is operated by the oil pressure from the 2-3/4-3 shift valve 57, thus applying the oil pressure to the rear clutch.
The hydraulic control device also includes an end clutch valve 58. The valve spool of the above valve 58 is operated by the 2nd or 3rd-stage pressure from the SCV or by the control pressure from the 2-3/4-3 shift valve 57, thus controlling the oil pressure for the end clutch.
The operation of the above hydraulic control device of the automatic transmission will be described hereinbelow, with the device performing an R-N-D shifting operation.
When the gear shifting lever is positioned in the R-range, the reverse-stage pressure from the manual valve 52 is applied to the regulator valve 51, thus allowing the regulating valve 51 to regulate the oil pressure. The oil pressure thus becomes slightly higher than a Drive-stage pressure and is suitable for a reverse stage. The regulated oil pressure of the regulator valve 51 is, thereafter, applied to both the SR part of the quick-down brake and the front clutch through the 2-3/4-3 shift valve 57. The regulated oil pressure is also applied to the low-reverse brake through in the order of the N-R control valve 55 and the 1-2 shift valve 56. Therefore, both the front clutch and the low-reverse brake are operated, while the change gear mechanism forms a speed change ratio suitable for the reverse stage.
When a driver shifts the gear shifting lever from the R-range into the D-range with the lever passing through the N-range, the manual valve 52 releases the pressure from the reverse-stage pressure passage, while providing a control pressure for the regulator valve 51, thus forming a line pressure. The manual valve 52 also provides a drive pressure for the hydraulic circuit.
When the reverse-stage pressure is released as described above, the pressurized oil of both the front clutch and the SR part of the quick-down brake is drained through in the order of the 2-3/4-3 shift valve 57 and an orifice "O". Meanwhile, the pressurized oil of the low-reverse brake in the above state is drained through the 1-2 shift valve 56.
When the gear shifting lever is positioned in the P, R or N-range, the SCSV-A is turned on with the SCSV-B being turned off. Therefore, the drive pressure from the manual valve 52 allows the SCV to instantaneously generate a 2nd-stage pressure. The 2nd-stage pressure from the SCV actuates both the 1-2 shift valve 56 and the end clutch valve 58.
Meanwhile, the TCU in the above state controls the PCSV so that the drive pressure from the manual valve 52 is controlled when it passes through the PCV prior to being applied to the rear clutch through the N-D control valve 54. The N-D control valve 54 also allows the drive pressure from the manual valve 52 to be applied to the SA part of the quick-down brake through the 1-2 shift valve 56.
Of course, when a predetermined time is elapsed after the starting of a gear shifting action, the increased oil pressure for the rear clutch operates the valve spool of the N-D control valve 54, thus allowing the pressure from the PCV to be applied to the SA part of the quick-down brake. On the other hand, the drive pressure from the manual valve 52 is also applied to the rear clutch through a check valve "C", which is connected to the N-D control valve 54 in parallel. In the above state, the TCU checks the shifting operation and gradually removes the 2nd-stage pressure by turning on the SCSV-A, thus releasing the SA pressure and setting the speed stage into the lst-stage of the D-range.
When the pressurized oil is drained from both the front clutch and the SR part of the quick-down brake through the 2-3/4-3 shift valve 57 in the R-N shifting operation, the oil is controlled by the orifice "O", thus being slowly drained without generating any shifting shock.
However, the above hydraulic control device is problematic in that the SA pressure may give a negative effect to the SR pressure during an R-N-D shifting operation, thus causing the SR pressure to partially remain in the SR part of the quick-down brake during the R-N-D shift operation. The remaining SR pressure acts on the front clutch, which communicates with the SR part of the quick-down brake. Therefore, the hydraulic control device unexpectedly allows both the front clutch and the rear clutch to be actuated at the same time, causing a tie-up shock. Due to such a tie-up shock, the hydraulic control device regrettably increases the shifting shock and deteriorates the shifting sense during such an R-N-D shifting operation.